Introduce `FromEntropy` trait to replace dummy values

[gavofyork]

Introduce a new trait available in std only:

#[cfg(feature = "std")]
pub trait FromEntropy {
  fn from_entropy(entropy: impl Encode) -> Self;
}

And implement it for AccountIds, Addresss, Signatures and everything for which it is useful to be able to generate dummy values deterministically.

This can replace dummy_ functions, Defaults and helpers such as fn account() used in tests and benchmarking.

[burdges]

How is this meant to be used?

[gavofyork]

/// Create random values of `Self` given a stream of entropy.
pub trait FromEntropy: Sized {
	/// Create a random value of `Self` given a stream of random bytes on `input`. May only fail if
	/// `input` has an error.
	fn from_entropy(input: &mut impl codec::Input) -> Result<Self, codec::Error>;
}

Basically if you supply an input of entropy, then for any type which implements this trait you'll be able to get an arbitrary but valid value.

This is different to just using Decode::decode since not that many types can be validly decoded from a random stream of bytes. E.g. for 254 of 256 possible byte values bool would fail to decode. So our impl for FromEntropy for bool is:

impl FromEntropy for bool {
	fn from_entropy(input: &mut impl codec::Input) -> Result<Self, codec::Error> {
		Ok(input.read_byte()? % 2 == 1)
	}
}

The simplesy way of using it would be to get a "default" value, e.g.: AccountId::from_entropy(TrailingZeroInput::zeros()). This will return a valid account ID value, which can be useful in tests. The zeros could be replaced by a (pseudo-) random input source in order to get some other value.